Development of reactively sputtered metal oxide films for hydrogen-producing hybrid multijunction photoelectrodes

The multijunction hybrid photoelectrode for hydrogen production developed at UH incorporates a metal-oxide photoelectrochemical top junction which is deposited onto an underlying solid state junction which generates additional voltage bias for efficient water splitting. Initial attempts to fabricate hybrid photoelectrodes using nano-structured iron-oxide films deposited by spray pyrolysis were largely unsuccessful because the pyrolysis temperatures of approximately 400 °C were high enough to cause significant damage to the underlying amorphous-silicon-based solid state junctions. This paper describes the development of low temperature (<200°C) reactively sputtered metal oxide films with properties specifically optimized for use in hybrid photoelectrode applications. The primary materials investigated include iron oxide, for use in alkaline photoelectrolysis, and tungsten trioxide, for use in acid photoelectrolysis. To date, the sputtered tungsten trioxide materials have demonstrated higher levels of photo-generated current compared with sputtered iron-oxide films. Initial hybrid photoelectrode devices fabricated using tungsten-trioxide films sputter deposited onto tandem amorphous-silicon junctions are described which exhibited stable solar-to-hydrogen conversion in acidic media at efficiencies up to 1% in outdoor tests. Plans to enhance efficiency by further oxide film optimizations, for example using surface texturing and possible doping, are also discussed.